U.S. patent number 3,720,285 [Application Number 05/060,188] was granted by the patent office on 1973-03-13 for loudspeakers.
This patent grant is currently assigned to The Rand Organisation Limited. Invention is credited to John D. Collinson, Kenneth F. Russell.
United States Patent |
3,720,285 |
Russell , et al. |
March 13, 1973 |
LOUDSPEAKERS
Abstract
An electro-acoustic transducer such as a loudspeaker is housed
in an enclosure having a filling comprising a rigid body of, for
example, foamed polyurethane which is capable of transmitting
pressure changes caused by operation of the transducer. The body
may have an air-impermeable surface to form a sealed enclosure.
Inventors: |
Russell; Kenneth F. (Shipley
Baildon, EN), Collinson; John D. (Otley,
EN) |
Assignee: |
The Rand Organisation Limited
(London, EN)
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Family
ID: |
22027917 |
Appl.
No.: |
05/060,188 |
Filed: |
June 30, 1970 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
Issue Date |
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788483 |
Jan 2, 1969 |
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Current U.S.
Class: |
181/151;
264/46.6; 264/46.7 |
Current CPC
Class: |
H04R
1/288 (20130101) |
Current International
Class: |
H04R
1/28 (20060101); G10k 013/00 (); H04r 001/28 () |
Field of
Search: |
;181/31R,31B,DIG.1 |
References Cited
[Referenced By]
U.S. Patent Documents
Foreign Patent Documents
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251,133 |
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Apr 1964 |
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AU |
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697,869 |
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Nov 1964 |
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CA |
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1,326,414 |
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Apr 1963 |
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FR |
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Primary Examiner: Tomsky; Stephen J.
Parent Case Text
This application is a division of Ser. No. 788,483, now abandoned.
Claims
What is claimed is:
1. In combination:
an electroacoustic transducer,
an enclosure comprising a skin of plastic material and a filling
made of substantially the same plastic material as said skin formed
into a rigid cellular structure having a majority of
intercommunicating cells and defining a cavity in which said
electroacoustic transducer is mounted,
said skin having an aperture therein in a region in which sound
radiated from said transducer into said cavity impinges directly on
said filling.
2. The combination of claim 1 wherein said filling of rigid
cellular plastics material comprises foamed plastics material.
3. The combination of claim 2 wherein said filling of rigid
cellular plastics material comprises foamed polyurethane.
4. The combination of claim 1 wherein said skin covers the entire
outer surface of said rigid cellular plastics material with the
exception of the region of said aperture.
5. The combination of claim 1 wherein said filling of rigid
cellular plastics material has a recess for receiving said
electro-acoustic transducer.
6. The combination of claim 5 wherein said electro-acoustic
transducer is sealingly connected to said skin at said aperture
whereby the space within said enclosure behind said
electro-acoustic transducer is substantially sealed.
7. The combination of claim 1 further including means securing said
transducer in said enclosure comprising transducer support members
embedded in said filling of rigid cellular plastics material.
Description
This invention relates to enclosures for electro-acoustic
transducers such as loudspeakers capable of producing from an
electric information-carrying signal, sound waves carrying
information corresponding to that in the signal.
The invention is particularly applicable to loudspeakers in which
sound waves are generated by a rigid diaphragm urged into motion as
a whole by an electro-magnetic system which is energized by the
signal and a part of which is coupled to the diaphragm. The
invention is, however, also applicable to alternative forms of
loudspeakers, for example loudspeakers of the kind in which the
sound waves are generated by an electric field produced by the
signal between electrodes.
With the exception of the case where they are freely mounted,
loudspeakers have hitherto been mounted within an enclosure or
cabinet having walls provided with a suitable decorative external
finish.
It is essential, for high quality sound reproduction, that the
sound waves radiated by the assembly comprising the combination of
a transducer such as a loudspeaker and the enclosure in which it is
mounted should bear the closest possible correspondence to the
information contained in the electrical signal applied to the
transducer. Any deviation between the radiated sound waves and the
information in the signal reduces the faithfulness of the
reproduction and is known as distortion or coloring.
From inter alia an acoustic point of view, the walls of such an
enclosure have hitherto been necessary not only to provide support
for the loudspeaker but also to prevent unrestricted pressure
changes within the enclosure being transmitted to the surrounding
air and vice versa. However, such walls are themselves urged into
vibration by the sound waves generated in the loudspeaker and this
vibration of the walls consequently distorts the sound radiated by
the assembly. The volume of air within the enclosure, by virtue of
its properties, also is urged into complex vibration and further
distorts the sound radiated by the speaker-enclosure assembly, by
virtue of the resonances which are excited by the sound energy.
In order to overcome these difficulties, it has been common
practice to construct the walls of the enclosure of heavy panels
which resist vibration and sometimes additionally to employ
internal bracing together with lining of the panels with a suitable
damping material.
In addition and in order to modify the acoustic properties of the
space within the enclosure, it has been common practice also to
fully or to partly fill the enclosure with a light sound absorbent
material. This absorbent material assists in absorbing standing
wave energy resulting from reflecting of sound waves between the
sides of the enclosure and the use of such material reduces the
coloration of sound which results from standing waves. The
absorbent material generally used is glass fiber and the like which
must be suitably supported within the enclosure into which it is
introduced.
According to one aspect of the present invention, an enclosure for
an electric transducer such as a loudspeaker has a filling
comprising a rigid body of having substantially the same shape as
the enclosure and including a matrix of passages or
intercommunicating cavities which are capable of transmitting
through the body, gas pressure changes caused by the
transducer.
The body is preferably suitably shaped to receive at least part of
the transducer, for example by being provided with a suitably
shaped depression.
Conveniently the body is a block of rigid cellular material in
which the majority of the cells are intercommunicating, so that the
pressure changes produced by the transducer can be transmitted
through the body. A suitable cellular material is a foamed plastics
material such as polyurethane which is produced by means well known
in the art.
Conveniently the body comprising the enclosure is produced by
inserting a fluid plastics material together with a suitable
foaming agent into a mold having internal dimensions substantially
that of the enclosure, so that the foamed plastics material fills
and sets in the mould. In this case the mould may be arranged
simultaneously to provide the body with one or more depressions
respectively for receiving one or more transducers. The mould may
also be arranged to support, in the correct position, support means
by which one or more transducers may be secured to the body, so
that these support means become firmly and rigidly incorporated
within the body after moulding is completed. The securing means may
for example, comprise a baffle to which the transducer is
subsequently attached.
Alternatively, the moulding may be made with the transducer already
in situ within the mould so that the transducer becomes securely
incorporated in and rigidly secured to, the body. In this case,
provision must be made for preventing the expanding foaming
material from interfering with the transducer, for example, with
the diaphragm of a loudspeaker, and subsequently preventing its
excursions.
Alternatively the body is produced by suitably bonding together
particles of a sound absorbing material so that the matrix is
produced by the communicating interstices between the particles. In
this case the bonding may also be accomplished in a suitable mould
and may be accompanied by some compaction of the particles.
In a preferred embodiment of the invention, the whole of the outer
surface of the enclosure with the exception of the orifice through
which, in use of the enclosure, sound generated by the transducer
is directly transmitted, is arranged to be air-impermeable to
produce what is termed a "sealed" enclosure. In such a sealed
enclosure, the gas conditions which are brought about by the matrix
are isothermal so that the different velocity of sound within the
enclosure, as compared with that external to the enclosure (i.e. in
the atmosphere), where conditions are adiabatic, is equivalent to
increasing the volume of the enclosure. The compliance of the
transducer assembly is thus increased and further improves the low
resonant frequency and the Q-characteristics which are provided by
the enclosure of the invention.
An air-impermeable outer surface on the body may be produced by
suitably moulding the body so that an outer impermeable skin is
formed. This skin need not be thick relatively to the dimensions of
the body, the rigidity of the body being sufficient to give the
necessary rigidity and support to the skin. It will be appreciated
that with this technique, where a depression effective to receive
the transducer is moulded integrally with the body an
air-impermeable skin formed over the surface of the depression must
be separately removed after moulding, to allow pressure changes
produced by the transducer to be transmitted into and through the
body.
A sealed enclosure may also be produced by covering the body with a
suitable material differing from that of the body. Suitably, the
body is covered with an impermeable sheet material, such as wood,
metal or plastics sheet material, suitably treated if desired to
produce an external decorative effect.
Where the rigid body in the enclosure is moulded, the
air-impermeable sheet material may itself be arranged to form the
mould. By this means the foamed material will, in general, after
expanding to fill and set in the mould, become firmly bonded to the
sheet material thereby giving it further support and rigidity, and
enabling the sheet material of relatively low inherent rigidity to
be employed.
In one embodiment of the invention, a sealed enclosure is provided
with a suitable opening or resistive window to allow controlled air
flow in response to pressure differences between the interior of
the enclosure and the atmosphere, and thereby to enable the
enclosure respectively to act either as a resonator or to enable
its resonance characteristics to be modified.
Such an opening may be incorporated into the skin of the body
during moulding or may be suitably incorporated into the sheet
covering material when this is used to form the mould itself.
It will be appreciated that where a sealed enclosure is employed,
it is essential that the transducer itself forms an airtight seal
with the orifice from which sound waves, generated by the
transducer are directly radiated. The moulding of the transducer or
the supporting baffle into the body is accordingly arranged to
ensure that such a seal is produced.
Where the body in the enclosure is merely provided with an
air-impermeable skin, the transducer itself may be placed in a
suitable position in the mould, the rear of the transducer being
covered with a mesh, so that the foamed material will form around
the transducer, for example, a loudspeaker unit without interfering
with the transducer, but at the same time will mould sufficiently
around the supporting means of the transducer (i.e., the frame of a
loudspeaker) to hold this rigidly in place. An inhibiting agent on
the wire mesh will prevent the foamed material from forming an
impermeable skin behind the transducer, i.e., behind the diaphragm
of a loudspeaker.
Alternatively, where the body is moulded to include a depression
for receiving the transducer, the body may be moulded to include
fixing studs, by which the transducer may be held within the body
with sufficient rigidity. If during the moulding an air-impermeable
skin is formed over the surface of the depression, then this may
separately be broken to allow the gas pressure changes produced by
the transducer to be transmitted through the body.
In the case where the enclosure is arranged to incorporate more
than one transducer, each of which is arranged to reproduce a
different frequency range, the lower frequency transducer is
mounted in any of the ways herein before described. However, the
other higher frequency transducer or transducers preferably are
incorporated into the rigid body after a seal is incorporated to
prevent air pressure changes generated by them from having access
to the matrix within the body. This seal can be formed by the skin
produced on the surface of the corresponding depression when this
is moulded integrally with the body. Alternatively, such higher
frequency transducers can be directly moulded into the body of the
enclosure by allowing the material to foam around them.
The depressions, in the body can, if desired contain an
accoustically absorbent material.
Where the enclosure is arranged to receive more than one
transducer, each of which is arranged to reproduce sound within a
different frequency range, a network effective to divide from the
electrical signal driving the transducers, the frequency range
components driving the respective transducers can also be
incorporated into the body and preferably, is moulded directly into
the body of the enclosure.
In one embodiment of the invention open cell expanded material can
also be used to absorb the sound radiated from the rear of a
transducer.
In an alternative embodiment of the invention the body in the
enclosure is in elongated form with an internal cavity which is
open at one of the ends of the body to receive the transducer.
In this arrangement, the body is arranged so that substantially the
whole of the sound energy radiated from the rear end of the
transducer is absorbed in the cavity in order to reduce to a
minimum the distortion produced by the transducer-enclosure
combination.
As before the body is preferably provided with an air-impermeable
outer surface layer to prevent unrestricted transfer of gas between
the inside of the enclosure and the surrounding atmosphere.
Preferably the body is moulded and the skin is provided on the
outer surface of the cavity during the moulding operation.
Embodiments of the invention will now be particularly described, by
way of example, with reference to accompanying drawings, in
which:
FIG. 1 is a schematic partly cutaway perspective view of one
embodiment of a loudspeaker enclosure according to the
invention,
FIG. 2 is a partial sectional view of an alternative embodiment of
the enclosure shown in FIG. 1 and:
FIG. 3 is an axial section of a further embodiment of a loudspeaker
enclosure according to the invention.
Referring to FIG. 1 of the drawing this shows a loudspeaker cabinet
in the form of a rectangular enclosure with an internal filling
comprising a rigid body 2 of foamed polyurethane covered by a
decorative plastics laminate sheet material 4. The sheet material 4
is contiguous with and preferably bonded to, the body 2 with the
exception of the area of an orifice formed by a central depression
6. This depression 6 which is provided in one face of the body 2 is
effective to receive a loudspeaker 10 (FIG. 2) which is firmly
secured to the enclosure by way of mounting studs 8 embedded in the
body 2.
Where the periphery of the loudspeaker frame, (as shown in FIG. 2),
makes a gas tight seal with the edge of the orifice formed by the
depression 6, the enclosure will be a sealed enclosure of the kind
thereinbefore referred to, if the sheet material of the loudspeaker
diaphragm 12 is unbroken and air-impermeable.
The enclosure of FIG. 1 may conveniently be produced by assembling
the sheet material 4 to the dimensions shown in order to produce a
mould.
A male mandrel (not shown) having the shape of the depression 6 is
arranged releasably to hold the loudspeaker fixing studs 8 and is
introduced into the orifice provided in the sheet material 4 at its
central region. The mandrel is preferably coated with a release
agent to facilitate its subsequent removal from the mould.
Fluid polyurethane together with a foaming agent is then introduced
into the mould so as to foam and fill the space within the mould
and subsequently to set, bonding to the sheet material 4 of the
mould. The conditions under which the polyurethane is foamed are
arranged to produce a majority of inter-communicating cells within
the foamed material, to enable gas pressure changes generated by
the excursions of the loudspeaker diaphragm 12 to be transmitted
through the body 2. The formation of an air-impermeable skin on the
surface of the depression 6 must be prevented during moulding, as
hereinafter described, or, if such a skin is formed, it must
subsequently be removed.
In the embodiment of the invention described with reference to FIG.
1 and 2, the gas conditions within the matrix of the body of the
enclosure become isothermal to cause a reduction in the velocity of
sound compared with the adiabatic conditions existing in the
surrounding atmosphere. The change to isothermal conditions has the
effect of increasing the effective volume of the enclosure and
makes possible the use of enclosures of relatively smaller volumes
which retain acoustic characteristics normally capable of being
provided by enclosures of relatively larger volume.
After solidification of the body 2 the mould is released from the
studs 8 which are left firmly embedded in the body 2, now provided
with the depression 6 for receiving the loudspeaker 10. The
loudspeaker 10 may be secured to the studs 8 by any suitable
means.
Any bonding of the foamed body 2 to the sheet material 4 will, of
course, increase the overall strength of the enclosure and enable
sheet material 4 of low inherent rigidity to be employed.
In an alternative embodiment of the invention shown in FIG. 2, the
depression-forming mandrel is dispensed with, and the loudspeaker
10 itself is inserted in the mould formed by the sheet material 4,
so that the periphery of the loudspeaker frame closely abuts the
periphery of the orifice 14 in the mould.
A gauze 16 coated with an agent effective to inhibit formation of
an air-impermeable skin on foamed polyurethane, is disposed at the
rear of the loudspeaker to form a space into which the polyurethane
cannot foam. This clear space prevents any interference with the
free excursion of the loudspeaker diaphragm 12.
The moulding is completed by inserting fluid polyurethane as
before, together with a foaming agent in the mould. The foamed
material expands to form a gas-tight seal between the periphery of
the loudspeaker frame and the orifice 14 to permit the enclosure to
become a sealed enclosure as hereinbefore defined.
In addition and in order to modify the acoustic properties of the
space within the enclosure, it has been common practice also to
fully or to partly fill the enclosure with a light sound-absorbent
material. This absorbent material assists in absorbing the energy
produced by the rear of the loudspeaker diaphragm and also standing
wave energy resulting from the internal reflection of sound waves
from the sides of the enclosure. This prevents a substantial part
of this energy from being transmitted through the openings in the
cabinet to the listener. The use of such sound-absorbent material
therefore reduces the coloration of sound. A second reason for
filling or partly filling an enclosure with sound-absorbent
material is to change the gas conditions in the enclosure from
adiabatic to isothermal. This causes a reduction in the velocity of
sound within the enclosure as compared with that externally of the
enclosure, which in turn results in an increase in the compliance
of the loudspeaker-enclosure system, equivalent to a substantial
increase in the cabinet volume. This enhances the low frequency
performance and also lowers the Q of the system. The
sound-absorbent material generally used is fiber-glass and the like
which must be suitably supported within the enclosure into which it
is introduced.
A further embodiment of the invention is shown in FIG. 3, which is
particularly applicable to loudspeakers operating in what is
usually regarded as the mid-frequency range. In this embodiment the
enclosure is elongated and its walls 20 comprise an impermeable
skin on the foamed material 2 or added sheets of impermeable
material. The foamed material of the body 2 is effective in
absorbing all or substantially all the energy. The speaker 10a is
supported by support means 21 in the internal cavity 22 in the
foamed material, and sealed to the orifice in the skin, as
described in greater detail in the introductory part of this
specification.
The preferred cellular material for the body 2 has been found to be
rigid foamed polyurethane having a density of about 1 pound per
cubic foot and having about 90 percent of its internal cells
inter-communicating.
It will be appreciated that, while the invention has been described
with particular reference to foamed polyurethane it is equally
applicable to any material or plastics material in which a matrix
of inter-communicating passages or cells are provided to transmit
through the body air pressure changes generated by the loudspeaker
or alternative form of transducer.
* * * * *